1. Field of the Invention
This invention relates to modulation of optical carriers.
2. Description of the Related Art
Many conventional optical modulators implement binary ON/OFF keying modulation schemes. The binary ON/OFF keying modulation schemes encode one data bit onto an optical carrier per coding interval. While such modulation schemes are straightforward to implement, it is often desirable to encode more than one data bit onto the optical carrier per coding interval, e.g., to support a higher data rate.
Other conventional optical modulators implement a quadrature phase shift keying (QPSK) modulation scheme. The QPSK modulation scheme encodes two data bits onto an optical carrier per symbol interval thereby producing the constellation of signal points shown in FIG. 1. In the QPSK constellation, each signal point has x and y components of the same magnitude. The various signal points of the QPSK constellation are related by reflections about the x-axis and/or the y-axis. In the QPSK modulation scheme, the optical carrier's in-phase and quadrature-phase components represent the x-coordinates and the y-coordinates of the signal points.
The conventional QPSK optical modulator includes a Mach-Zehnder interferometer (MZI). The MZI has two arms whose optical path lengths differ by ¼ of the optical carrier's wavelength up to integer multiples of the wavelength. Each arm of the MZI includes an electro-optically controlled phase shifter, i.e., an MZI. The phase shifters generate phase shifts of 0 or π on the optical carrier in response to the binary voltage values of the digital signals being encoded. In the QPSK optical modulator, one arm of the MZI encodes one data bit onto the sign of the in-phase component of the optical carrier, and the other arm of the MZI encodes one data bit onto the sign of the quadrature-phase component of the optical carrier.